Use of hydrate slurry for transport of associated gas

ABSTRACT

A system and method for the transport of the natural gas associated with the production of crude oil from offshore wells includes transforming the associated gas into a solid gas hydrate. The solid gas hydrate is mixed with liquid crude oil to form a pumpable slurry. The conversion of the gas into a solid gas hydrate and the mixing of the pumpable slurry may take place on a floating offshore storage vessel or on a shuttle tanker. The pumpable slurry is transported on a shuttle tanker to an onshore location where the solid gas hydrate may be extracted from the oil and the associated gas transformed from a solid gas hydrate back into a gaseous state.

This patent application claims the benefit of Provisional U.S. PatentApplication No. 60/545,252 filed Feb. 17, 2004.

FIELD

This invention relates to a system and method for transportingassociated natural gas produced at an offshore location to an onshorelocation; more particularly, this invention relates to the transport ofassociated natural gas from an offshore production or storage facilityto an onshore storage facility or a refinery using shuttle tankers.

BACKGROUND

In locations such as the Gulf of Mexico, liquid and gaseous hydrocarbonsproduced at offshore locations are transported from offshore locationsto onshore facilities using pipelines laid along the ocean floor.However, the cost of laying a pipeline on the ocean floor to remote deepwater locations can become prohibitively expensive. Accordingly, theoperators of offshore production platforms located in remote deep waterlocations must use other means of transport to move liquid and gaseoushydrocarbons to an onshore location for storage or further processing.One such system involves the use of shuttle tankers. Shuttle tankershave been used in the North Sea for many years to transport hydrocarbonsproduced from offshore wells to onshore locations.

The transportation of gaseous hydrocarbons is particularly problematic.A variety of techniques have been tried from transporting gaseoushydrocarbons in a highly compressed state to transporting the gaseoushydrocarbons in liquid form. Both such techniques require specialequipment to either compress or liquify the natural gas. Once compressedor liquified, special containment facilities are needed to maintain thenatural gas in a compressed or liquified state. The containmentfacilities must be made of specialty steels and other expensivematerials to withstand the extremely cold temperatures of whichcompressed or liquified natural gas must be transported. Accordingly,the cost of the equipment to transport natural gas from offshoreproduction platforms to onshore locations is so high that many operatorsof offshore production platforms simply prefer to flare or burn off thenatural gas associated with the production of crude oil, if allowed todo so by host governments.

Accordingly, a need remains in the art for a system and method forinexpensive transport of natural gas from offshore production platformsto onshore locations without the need for special equipment dedicatedsolely to the transport of natural gas.

SUMMARY

The system and method of the present invention provides for theinexpensive transport of natural gas from offshore production platformsto onshore locations without the need for special equipment dedicatedsolely to the transport of natural gas.

The system and method for the transport of associated gas from anoffshore production platform to an onshore facility of the presentinvention includes a plant for transforming the associated gas intosolid gas hydrates. This solid gas hydrates are then mixed with liquidcrude oil to create a pumpable slurry. The pumpable slurry istransported to an onshore location using a shuttle tanker. At theonshore location, the solid gas hydrates are separated from the liquidcrude oil and transformed back into a gaseous state for storage orfurther processing.

The plant for transforming the associated gas into solid gas hydratesmay be located either on a floating offshore storage vessel or on ashuttle tanker calling on the onshore location. If the plant fortransforming associated gas into solid gas hydrates is located on thefloating offshore vessel, the solid gas hydrates are mixed with liquidoil on the floating offshore storage vessel to form a pumpable slurry.The pumpable slurry is then transferred to a shuttle tanker. Once theshuttle tanker reaches the onshore location, the pumpable slurry isoffloaded from the shuttle tanker. At the onshore location, the solidgas hydrate and the liquid crude oil are separated so that the solid gashydrate can be transformed back to a gaseous state.

If the plant for transforming an associated gas into solid gas hydratesis located on the shuttle tanker, the solid gas hydrates are mixed withliquid crude on the shuttle tanker to form a pumpable slurry.

To assure that the solid gas hydrate remains in a solid form while inthe pumpable slurry, it is necessary to cool the tank in which thepumpable slurry is stored, whether the tank is located onboard thefloating offshore storage vessel or onboard the shuttle tanker.

BRIEF DESCRIPTION OF THE DRAWING FIGURES

A better understanding of the system and method of the present inventionmay be had by reference to the drawing figures wherein:

FIG. 1 is a flow chart of the system and method of the present inventionwherein the associated gas is transformed into a solid gas hydrateonboard a floating storage vessel;

FIG. 2 is a flow chart of the system and method of the present inventionwherein the associated gas is transformed into a solid gas hydrateonboard a shuttle tanker.

DESCRIPTION OF THE EMBODIMENTS

The system and method of the present invention 10, 110 are bestunderstood by reference to FIG. 1 and FIG. 2 against the background ofthe need to transport the gaseous hydrocarbons associated with theliquid crude oil produced by an offshore platform from a subseahydrocarbon reservoir.

The offshore platform is typically located at a location which is inrelatively close proximity to the subsea reservoir from which crude oiland associated gas are extracted. The liquid crude oil and theassociated gas are usually separated one from another at the offshoreplatform 12 and then further transferred from the platform to a floatingstorage vessel (FSO) 14 by a variety of different means. Such transfermeans include a collection of conduits, pumps, valves, and connectorswell known to those of ordinary skill in the art.

Onboard the floating storage vessel, the natural gas may be temporarilystored 16 in a compressed state.

Because the floating storage vessel remains in a relatively stationarylocation offshore, the liquid crude oil and the associated gas must betransported onshore to a storage facility or to a refinery. According tothe disclosed system, this transfer is accomplished by moving the crudeoil and the gas from the floating offshore storage vessel to a shuttletanker 18. The shuttle tanker carries both the liquid and gashydrocarbons to an onshore location.

More specifically, the term offshore production platform is used aboveto describe the starting point for the oil and gas removed from a subseareservoir. Those of ordinary skill in the art will realize that the termoffshore product platform encompasses a variety of differentstructures—floating, fixed, or some combination thereof. The offshoreproduction platform is equipped to separate the liquid hydrocarbons,generally called crude oil, from the gaseous hydrocarbons, generallycalled natural gas.

Since most offshore production platforms do not include storage foreither crude oil or natural gas, the crude oil and natural gas must betransferred into a system which conveys it away from the offshoreproduction platform. One such system is a floating storage vessel. Thefloating storage vessel receives the crude oil from the offshoreproduction platform. This oil is often called “dead crude oil.” Thefloating storage vessel also receives the associated natural gasseparately 14.

According to the present invention, the associated natural gas isconverted into a solid gas hydrate 20 (a form of snow) by a plant on thefloating storage vessel. The plant employs a process of controlledreduction in temperature, controlled pressure, and controlledintroduction of water. Such plants are well known to those of ordinaryskill in the art. Once the natural gas is converted into a solid gashydrate, it is mixed 22 with the liquid dead crude oil to form apumpable slurry. It has been found that lighter weight dead crude oil isbest used for creation of the pumpable slurry. Such lighter weight deadcrude oil is typically found near the top of oil storage tanks.

Those with experience in creating a pumpable slurry of solid gas hydrateand dead crude oil have found that a gas-to-oil ratio in the range ofabout 200 scf/bbl to about 600 scf/bbl is acceptable.

As shown in FIG. 1, following the creation of the pumpable slurryonboard the floating storage vessel, it may then be transferred to ashuttle tanker 18 using a conventional conduit system. It has been foundthat if the tanks containing the pumpable slurry are kept cool; that is,in a temperature range of about 0° F. to about −10° F., the gas hydrateswill remain in their solid form. In the temperature range of 0° F. toabout −10° F., no special cryogenic materials are required.Specifically, shuttle tankers adapted for transport of the pumpableslurry are made from the same materials as other ships that might beused in cold weather climates, such as near the Arctic Circle.

The shuttle tankers transport the pumpable slurry to an onshorelocation. At the onshore location, the pumpable slurry is offloaded fromthe shuttle tanker 24. It is at the onshore location that the solid gashydrate may be separated from the pumpable slurry and then furthertransformed back into natural gas 24.

In an alternate embodiment, and as shown in FIG. 2, the associatednatural gas is not transformed into a solid gas hydrate onboard thefloating offshore storage vessel but, instead, is transformed into asolid gas hydrate 120 and mixed into a pumpable slurry 122 onboard theshuttle tanker. In this case, the associated natural gas is received onthe floating offshore storage vessel in its gaseous state andtransferred to the shuttle tanker in its gaseous state where it may betemporarily stored 116. Alternatively, the associated natural as may betransferred directly to the shuttle tanker from the floating offshorestorage vessel without any temporary storage.

It is well known that natural gas can be transformed into a fuel whichcan be used as an energy source for engines or power equipment. Suchengines or power equipment transform the energy contained in the naturalgas into mechanical or electrical power. According to the instantinvention, a portion of the natural gas may be diverted from flow to thesolid gas hydrate plant to be transformed into mechanical or electricalpower for the floating storage vessel or the shuttle tanker 90.

The process for making a pumpable slurry of a solid gas hydrate andliquid crude oil is known in the art. Specifically, produced liquid oiland associated natural gas is conveyed to a separation module where theliquid oil is separated from the natural gas. The first stage of suchseparation modules operates at pressures above 750 psi.

The liquid crude oil, which is typically in an emulsion with water, isprocessed according to the following steps:

-   -   1. The crude oil-water emulsion is treated with either chemical        de-emulsifiers or electrostatic separation techniques.    -   2. The water and the crude oil are separated one from another.

The water from the crude oil water emulsion is transferred to a slopwater system. If needed, the water may also be treated to obtain adesired chemical profile. In some instances, the cleaned up water can bereturned to the sea.

Fresh water is typically used with associated natural gas to form ahydrate slurry, as the salt in sea water inhibits the formation of solidhydrates.

The gas from the separation module enters an LPG separation process toremove any rich LPG constituents and any condensates. This step chillsthe associated natural gas to about −50° F. and separates any condensedliquids from the methane, butane, and propane components of theassociated natural gas.

The remaining associated natural gas is conveyed to the plant whichmakes the solid gas hydrate. The gas is pumped into plant along withcold fresh water. The water is chilled to about 32° F. The combinationof the water with the gas forms a snow-like substance. Excess water isremoved from the plant.

The solid gas hydrate snow is combined with liquid crude oil to form apumpable slurry. This pumpable slurry is maintained at a temperaturefrom about 0° F. to about −10° F. at atmospheric pressure. Accordingly,if the pumpable slurry is to be maintained on a floating offshorestorage vessel, the storage tanks must be made of materials suited tosuch temperatures, and a cooling system is required to maintain thepumpable slurry in a temperature range from about 0° F. to about −10° F.Similarly, if the pumpable slurry is to be maintained on a shuttletanker, the storage tanks must be made of materials suited to suchtemperatures, and a cooling system is required to maintain the pumpableslurry in a temperature range from about 0° F. to about −10° F.

The advantage of the disclosed system and method is that a solid gashydrate can be mixed with liquid crude oil to form a pumpable slurrythat is easily and conveniently transported from an offshore productionplatform to an onshore location. The addition of a solid gas hydrate toliquid crude oil only slightly increases the volume of materialtransported. Only minimal changes are needed to convert the vessels usedto transport the pumpable slurry of a solid gas hydrate and crude oil.No special natural gas transport equipment is required. If there is nonatural gas to be transported, the shuttle tanker is simply used totransport liquid crude oil or other alternate uses falling within thecapabilities of the shuttle tanker.

While the present invention has been disclosed according to itspreferred embodiment, those of ordinary skill in the art will understandthat other embodiments have also been enabled. Such other embodimentswill be well understood by those of ordinary skill in the art.

1. A method for transporting the associated hydrocarbon gas produced atoffshore production platforms to an onshore location, said methodcomprising the steps of: transferring the associated gas to a floatingstorage vessel; transforming the associated gas into a solid gas hydrateonboard said floating storage vessel; forming a pumpable slurry bymixing said solid gas hydrate with liquid oil onboard said floatingstorage vessel; temporarily storing said pumpable slurry on saidfloating storage vessel; transferring said pumpable slurry from saidfloating storage vessel to shuttle tankers; transporting said pumpableslurry to the onshore location.
 2. The method as defined in claim 1further including the step of holding said pumpable slurry in cooledtanks while aboard said shuttle tanker.
 3. The method as defined inclaim 1 wherein a portion of said associated gas is used as an energysource onboard said floating storage vessel before being transformedinto a solid gas hydrate.
 4. The method as defined in claim 1 whereinsaid pumpable slurry is offloaded from said shuttle tanker at theonshore location.
 5. The method as defined in claim 1 wherein said solidgas hydrate is separated from said pumpable slurry at the onshorelocation and then transformed into gaseous hydrocarbons.
 6. A method fortransporting the associated hydrocarbon gas produced at offshoreproduction platforms to an onshore location, said method comprising thesteps of: transferring the associated gas to a floating storage vessel;transferring the associated gas from said floating storage vessel to ashuttle tanker; transforming the associated gas into a solid gas hydrateonboard said shuttle tanker; forming a pumpable slurry by mixing saidsolid gas hydrate with liquid oil onboard said shuttle tanker;transporting said pumpable slurry to the onshore location.
 7. The methodas defined in claim 1 wherein said pumpable slurry is held in cooledtanks while onboard said shuttle tanker.
 8. The method as defined inclaim 1 wherein a portion of said associated gas is used as an energysource onboard said shuttle tanker before being transformed into a solidgas hydrate.
 9. The method as defined in claim 1 wherein said pumpableslurry is offloaded from said shuttle tanker at the onshore location.10. The method as defined in claim 1 wherein said solid gas hydrate isseparated from said pumpable slurry at the onshore location and thentransformed into gaseous hydrocarbons.
 11. A system for transportingassociated hydrocarbon gas from an offshore production platform to anonshore location, said system comprising: a conduit for transferring gasfrom the offshore production platform to a floating storage vessel; aplant for transforming the associated gas into a solid gas hydrateonboard said floating storage vessel; a device for mixing said solid gashydrate with liquid crude oil contained on said floating storage vesselto form a pumpable slurry; a cooled tank on said floating storage vesselfor containing said pumpable slurry; a conduit for transferring saidpumpable slurry from said floating storage vessel to a shuttle tankercalling on the onshore location.
 12. The system as defined in claim 11further including a cooled tank onboard said shuttle tanker forcontaining said pumpable slurry.
 13. The system as defined in claim 11further including a conduit for diverting a portion of the associatedgas away from said plant and into an energy transformation systemonboard said floating storage vessel.
 14. The system as defined in claim11 further including a conduit to unload said pumpable slurry at theonshore location.
 15. The system as defined in claim 11 furtherincluding a plant to separate the solid gas hydrate from the pumpableslurry at the onshore location.
 16. A system for transporting associatedhydrocarbon gas from an offshore production platform to an onshorelocation, said system comprising: a conduit for transferring theassociated gas from the offshore production platform to a floatingstorage vessel; a conduit for transferring the associated gas from saidoffshore production vessel to a shuttle tanker; a plant for transformingthe associated gas into a solid gas hydrate onboard said shuttle tanker;a device for mixing said solid gas hydrate with liquid crude oilcontained on said shuttle tanker to form a pumpable slurry; a cooledtank on said shuttle tanker for containing said pumpable slurry; aconduit for transferring said pumpable slurry from said shuttle tankerat the onshore location.
 17. The system as defined in claim 16 furtherincluding a cooled tank onboard said floating storage vessel.
 18. Thesystem as defined in claim 16 further including a conduit for divertinga portion of the associated gas away from said plant and into an energytransformation system on said shuttle tanker.
 19. The system as definedin claim 16 further including a plant to separate the solid gas hydratefrom the pumpable slurry at the onshore location.